Electronic rotor pointing with high angular resolution

ABSTRACT

A laser level with a sensorless DC motor controller having a feedback sampling circuit connected in parallel with the coil to sense back EMF. An integrator provides a feedback signal from the sensed back EMF, which the amplified difference from a reference level is used as a pulse width control signal. A pulse width modulation generator uses the control signal to generate variable “on” time pulse widths for each motor drive pulse such that high resolution is provided to the DC motor, permitting accurate rotor position at low rotational speeds. It is emphasized that this abstract is provided to comply with the rules requiring an abstract to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that is will not be used to interpret or limit the scope or meaning of the claims.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/179,114 filed Jun. 25, 2002 now U.S. Pat. No. 6,762,575.

BACKGROUND OF THE INVENTION

The present invention relates generally to the control of DC motors, andparticularly, but not exclusively, to a method and apparatus providingelectronic rotor pointing with high angular resolution using asensorless permanent magnet DC motor.

There are numerous techniques in the prior art for controlling thepositioning of a rotor of a permanent magnet (PM) DC motor. Thesetechniques can be generalized into two major categories. The firstcategory generally includes those techniques in which a PM DC motor isdesigned to move at a precise speed in synchronism with, or locked to,the waveform of the driving voltage or current which energizes thewindings. PM stepper motors, which have a permanent magnet in the formof a rotor magnetized in alternate polarity “stripes” parallel to therotor shaft, are used with this type of technique. The step size(angular resolution) of such a motor is entirely a function of theangular “width” of these magnetized stripes, and an angular resolutionof 7.5 degrees is common in most inexpensive motors. However, costsincrease greatly if such motors are to provide high resolution in the 2to 5 arc minute range. Additionally, PM stepper motors move in steps bysending pulse trains of varying polarity to multiple windings. Thefrequency of the pulses and the phasing between the pulses applied tothe various windings determines the speed and direction of motor motion,respectively. As a result, precise control over rotor positioning isprovided, but at the cost of control circuitry complexity.

The second category of PM DC motor rotor positioning techniques includesthose in which sensors are external to, or built into, the motor.Typically, such position sensors include Hall effect sensors and opticalencoders. With Hall effect sensors, resolution is limited by the number,the positioning accuracy, and the gain tolerance of the sensors. Withoptical encoders, high resolution is provide at a higher cost. Opticalencoders require electronics for decoding and accumulation. Such asystem requires initialization, and over time may also require alignmentand adjustment. Accommodating either Hall effect sensors or opticalencoders also increases the size of the resulting device.

Conventional laser levels typically use the rotor of a sensorlesspermanent magnet DC motor to rotate a prism. The prism reflects a beamof laser light used in leveling operations. The operator controls such asensorless DC motor using open loop control (no feedback) and therefore,moves the beam at a desired speed by increasing or decreasing the DCvoltage applied to the motor. In order to position a beam in a desireddirection with such an arrangement, the operator typically jogs the DCmotor (applies a succession of voltage pulses) to point the laser beamat a small, distant object. However, with open loop control, rotorpointing by jogging the motor is inaccurate, often unrepeatable, and canbe frustrating to the operator due to the lack of reliable beampointing.

For example, stopping the laser beam on a two-inch wide object 100 feetaway requires a DC motor speed of about one revolution per minute givenan average human reaction time of 100 ms and a typical 6:1 drive ratio.However, rotating a DC motor at such slow speeds is problematic, since amotor's resolution changes with temperature and at different angularpositions due to unavoidable variations in the manufacturing process andwear patterns of a motor's bearings. These variations make finepositioning operations at such a slow speed difficult with open loopcontrol of a PM DC motor. However, due to space and cost considerations,using a stepper motor and/or position sensors in a laser level forclosed loop rotor positioning control of the DC motor to provideacceptable resolution is not economical.

Therefore, a method and apparatus are needed for providing electronicrotor pointing to a sensorless permanent magnet DC motor, which takeinto account friction, temperature, bearing manufacturing, and wearpattern variations to provide accurate, repeatable, and reliableresolution position control.

SUMMARY OF THE INVENTION

The present invention is a sensorless control method and circuit whichuses back electromagnetic force (EMF) as a feedback control to positiona beam of electromagnetic energy within a few arc-minutes of a desiredangular position. The system moves a sensorless PM DC motor in fineangular increments by applying short, high current drive pulses, whichovercome static friction and induce movement of the rotor. A measurementof the rotor's angular distance moved due to the drive pulses isobtained by integration of a sampled back EMF voltage generated by themotor in a time window following each pulse. This measurement of therotor's angular distance moved per pulse is used to control the pulsewidth or “on” time of the next drive pulse applied to the motor, therebyresulting in accurate, repeatable, and reliable fine positioningoperation of about 2 to 5 arc minutes.

Sensorless control systems, such as the type provided by the presentinvention, possess a number of advantages. Although the presentinvention is not limited to specific advantages or functionality, it isnoted these advantages include reduced component and sensor costs,reduced tooling and manufacture costs, improved reliability, andinvariance to changes in the operating environment and noise reduction.

In one aspect of the invention, one embodiment comprises a method forproviding improved angular resolution to a sensorless permanent magnetDC motor for rotor pointing. The method comprising supplying to themotor a pulse width modulated (PWM) motor drive pulse having an “on”time of a pulse width, and providing a sampling delay which preventssampling of inductive stored energy of the motor. The method furtherincludes providing after the sampling delay, a sampling window forsampling back EMF of the motor, and changing the pulse width of the “on”time of the motor drive pulse based on sampled back EMF in order toadjust rotor speed, thereby maintaining a set angular distance in a settime-period.

Another embodiment of the invention comprising a method of providingimproved angular resolution at rotational speeds below about 1 rpm to arotor of a sensorless permanent magnet DC motor used to move andposition a beam of electromagnetic energy. The method comprisessupplying to the motor a pulse width modulated (PWM) motor drive pulsehaving an “on” time of a pulse width which produces rotor motion, and an“off” time. The method further includes providing after a samplingdelay, a sampling window for sampling back EMF of the motor during the“off” time, and changing the pulse width of the “on” time of the motordrive pulse. The “on” time is based on the sampled back EMF and itspulse width is varied to adjust rotor speed and maintain a set angulardistance in a set time-period. The sampling delay prevents sampling ofinductive stored energy of the motor after expiration of the “on” time.

In another aspect of the present invention, one embodiment provides amotor controller for driving and providing close loop control of asensorless permanent magnet DC motor with improved angular resolutionfor rotor pointing. The motor controller comprises motor drive logicadapted to drive the motor with a series of motor drive pulses. Each ofthe motor drive pulses has an “on” time pulse width and an “off” timepulse width. Pulse width control logic is adapted to set the “on” timepulse width. The motor controller further includes feedback sample logicadapted to measure back EMF generated by the motor. The feedback samplelogic provides a sampling delay which prevents sampling of inductivestored energy of the motor after expiration of the “on” time pulsewidth, and a sampling window for sampling the back EMF of the motorduring the “off” time pulse width. The pulse width control logic isadapted to vary the “on” time pulse width of the motor drive pulse basedon sampled back EMF in order to adjust rotor speed and maintain a setangular distance in a set time-period.

Another embodiment of the invention comprises a motor controller adaptedto drive a sensorless permanent magnet DC motor and to provide improvedangular resolution for the motor rotor position. The motor controllercomprises a motor drive stage having gate actuator logic, power switchesadapted to energize at least one winding of the motor and beingcontrolled by the gate actuator logic with a series of drive pulses, anda frequency generator adapted to provide a timing signal to the actuatorlogic for modulation of the drive pulses. Each of the drive pulses has avariable “on” time and an “off” time. The motor controller furtherincludes a pulse width control stage that provides a control signal tothe gate actuator logic. The control signal sets a pulse width of the“on” time for each of the drive pulses. A feedback sampling stage isconnected in parallel with the motor winding and is adapted to sampleback EMF generated by the motor. The feedback sample stage has switchinglogic which provides a sampling delay to prevent sampling of inductivestored energy of the motor after expiration of the “on” time for each ofthe drive pulses. The feedback sample stage further includes samplebuffer logic which provides a sampling window for sampling the back EMFafter expiration of the sampling delay. The sampled back EMF isintegrated and compared to a reference to provide an input voltage tothe pulse width control stage based on the integrated back EMF. Thepulse width of the “on” time is set for each of the drive pulses inorder to adjust the rotor speed to maintain a set angular distance in aset time-period. This results in improved angular resolution.

In still another embodiment, a laser level having electronic rotorpointing with high angular resolution comprises a sensorless permanentmagnet DC motor, and a motor controller. The motor controller comprisesmotor drive logic that is adapted to drive the motor with a series ofmotor drive pulses, each motor drive pulse having an “on” time pulsewidth and an “off” time pulse width. The motor controller furtherincludes pulse width control logic adapted to set the “on” time pulsewidth, and feedback sample logic adapted to measure back EMF generatedby the motor. The feedback sample logic provides a sampling delay thatprevents sampling of inductive stored energy of the motor afterexpiration of the “on” time pulse width, and a sampling window forsampling the back EMF of the motor during the “off” time pulse width.The pulse width control logic is adapted to vary the “on” time pulsewidth of the motor drive pulse based on sampled back EMF. This adjustsrotor speed to maintain a set angular distance in a set time-period. Thelaser level further includes a power circuit for powering the laserlevel.

In yet another embodiment, a laser level has electronic rotor pointingwith high angular resolution. The laser level comprises a sensorlesspermanent magnet DC motor having a rotor adapted to rotate and positiona beam of laser light, and a motor controller adapted to drive thesensorless permanent magnet DC motor, thereby providing improved angularresolution for the rotor. The motor controller comprises a motor drivestage having gate actuator logic, and power switches adapted to energizeat least one winding of the motor. This results the gate actuator logiccontrolling the motor with a series of drive pulses. The motorcontroller further includes a frequency generator adapted to provide atiming signal to the actuator logic for modulation of the drive pulses,each the drive pulses having a variable “on” time and an “off” time. Apulse width control stage adapted to provide a control signal to thegate actuator logic. The control signal sets a pulse width of the “on”time for each of the drive pulses. The motor controller further includesa feedback sampling stage connected in parallel with the motor windingand adapted to sample back EMF generated by the motor. The feedbacksample stage has switching logic which provides a sampling delay toprevent sampling of inductive stored energy of the motor afterexpiration of the “on” time for each of the drive pulses, and a samplebuffer logic which provides a sampling window for sampling the back EMFafter expiration of the sampling delay. The sampled back EMF isintegrated and compared to a reference to provide an input voltage tothe pulse width control stage. The pulse width control stage sets thepulse width of the “on” time for each of the drive pulses in order toadjust the rotor speed and maintain a set angular distance in a settime-period, thereby providing the improved angular resolution. Thelaser level further includes a power circuit for powering the laserlevel.

These and other features and objects of the present invention will beapparent in light of the description of the invention embodied herein.It is noted that the scope of the claims is defined by the recitationstherein and not by the specific discussion of features and advantagesset forth in the present description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentinvention can be best understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a block diagram illustration of a motor controller suitablefor providing high angular resolution to a sensorless motor inaccordance with the present invention, the motor controller including afeedback sampling logic, a pulse width control logic, and a motor drivelogic;

FIG. 2 is a circuit diagram that illustrates an implementation of afeedback sampling logic according to the present invention;

FIG. 3 is a circuit diagram that illustrates an implementation of apulse width control logic according to the present invention; and

FIG. 4 illustrates representative signals associated with the motorcontroller for providing high angular resolution for a sensorless motoraccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a motor control system, generally indicated at 10,embodies aspects of the present invention. The motor control system 10includes a motor 12, and a motor controller 14. The motor 12 has astationary assembly, or stator, 16 and a rotatable assembly, or rotor,18 in magnetic coupling relation to the stator 16.

In the embodiments described herein, the motor 12 is a conventionalpermanent magnet DC motor. It is contemplated, however, that aspects ofthe present invention may be applicable to various electronicallycontrollable motors or dynamoelectric machines typically powered by anelectronic control circuit. Such motors include, for example, externalrotor motors (i.e., inside out motors), permanent magnet motors, singleand variable speed motors, selectable speed motors having a plurality ofspeeds, brushless DC motors, ironless rotor DC motors, “pancake” styleDC motors, electronically commutated motors, switched reluctance motorsand induction motors. Such motors may also provide one or more finite,discrete rotor speeds selected by an electrical switch or other controlcircuit.

As an example, motor 12 is a DC motor which has permanent magnetsmounted on its stator 16. As is known in the art, the rotor 18 of such amotor has one or more wire-wound coils, or windings. The winding orwindings on rotor 18, when energized with current, interact with thepermanent magnet stator 16 to produce positive or negative torque,depending on the direction of the current relative to the polarity ofthe magnets.

A motor shaft 20 mechanically connects rotor 18 to a particular deviceto be driven, such as a rotatable component 22. For example, therotatable component 22 comprises an electromagnetic beam source or aplatform for redirecting a beam of electromagnetic radiation such as alaser beam 24. The platform may include a prism or prism and/or one ormore mirrors for redirecting a beam of by reflection and/or refraction.Such a motor control system 10 may be included in a laser levelingsystem, generally indicated as 26.

Although the motor control system 10 is particularly useful for drivingand pointing the laser beam 24 of the laser leveling system 26 within afew arc-minutes of a desired angle, it is to be understood that motorcontrol system 10 may be part of a number of different systems fordriving other rotatable components. For example, rotatable component 22may be part of a servo system used to control the motion of a robotictargeting system of a surveying system, such as the type disclosed bycommonly assigned U.S. Pat. No. 6,035,254, the entire disclosure ofwhich is incorporated herein by reference. In addition, rotatablecomponent 22 may also include a connection mechanism 28 for coupling itto shaft 20. For example, the connection mechanism 28 may be a beltpulley system or a gearbox providing an increase or reduction in theangular velocity (i.e., RPMs) of the rotatable component 22.

A user interface or system control 30 provides system control signals tothe motor controller 14, via line 32. In one embodiment, the userinterface 30 is a set of contact switches and the system control signalstake the form of motor commands representing, for example, turn on andturn off commands, an increase speed command (fast), a rotationdirection command (i.e., clockwise or counterclockwise), and the like.In response to the system control signals, motor controller 14 thengenerates motor control signals. In other embodiments, the userinterface 30 may be electronically implemented with, or remotely from,the motor controller 14.

For manual positioning applications, to accommodate a human reactiontime of about 100 ms, the motor 12 needs to be rotated at a very lowspeed. In one embodiment, the available speed range for the motor isabout 0.5 RPM to about 50 RPM. The motor used in this embodiment willgenerate a back electromechanical force (EMF) of about 600 microvolts atabout 1 RPM. Other motors will generate a different value of back EMFwhich, as known by those skilled in the art, is the product of the backEMF constant for the specific type of motor used, and the motor's speed.

At very low speeds, the back EMF can be measured by briefly turning offthe motor's drive current and then measuring the terminal voltage afterthe inductive stored energy in the energized winding, or windings, hasdissipated. Applying short high current pulses creates the “off” periodsin which to measure the back EMF of motor 12. In addition to creatingthe “off” periods, applying short high current pulses also creates a“jolt” which breaks the rotor 18 free of static friction, and with eachpulse moves the rotor a small angular amount. Since the inductive storedenergy in the energized winding, or windings, can be dissipated muchmore quickly then the mechanical time constant due to inertia, the motorwill remain in motion while the back EMF is sampled during the “off”periods of the drive current pulse.

Integrating the sampled back EMF gives an indication of rotor speed,which is then used to adjust the pulse width (on-time) of the next drivecurrent pulse. Integrating the sampled back EMF has the advantage ofaveraging out “ringing” or oscillating voltages superimposed on the backEMF voltage. Such ringing may be caused by mechanical resonance orresonances in the rotor, the stator, the mounting component orcomponents, the connection mechanism to the rotatable component, therotatable component, and electrical resonance due to the interaction ofthe inductance of the windings and the capacitance in the motor drivecircuit.

As represented by the block diagram of FIG. 1, the above method ofsensorless DC motor control is implemented for illustration purposes inlogic stages of the motor controller 14. These logic stages includemotor drive logic 34, pulse width control logic 36, and feedbacksampling logic 38. It is to be appreciated that circuitry components,transistor logic, and programmable logic circuits, such as amicroprocessor or microcontroller and/or an application specificintegrated circuit (ASIC) or universal electronically commutated motorintegrated circuit (UECM IC), and combinations thereof, maybe used toimplement the following illustrative control circuit 14.

Pulse width control logic 36 provides, via line 40, a pulse widthcontrol signal to control electronically a plurality of gate actuators42. In turn, gate actuators 42 provide drive signals, via line 44, forswitching a plurality of power switches 46 (e.g., insulated gate bipolartransistors, bipolar junction transistors or metal oxide silicon fieldeffect transistors, etc.). It is to be appreciated that the pulse widthcontrol signal sets the “on” and “off” ratio for the drive signals,which are made variable depending on the results of feedback samplinglogic 38.

Varying the “on” time for each drive signal provides, at low rotor RPMs(e.g., 1 rpm), a simple, low cost, and efficient method of providinghigh resolution in rotor positioning. Since the motor 12 is driven witha pulse width modulation (PWM) signal, varying the duty cycle of thesignal will vary the drive time of the output. Therefore, by basing theduration of the drive time on the integration of the back EMF, the motor12 is locked into making fine speed changes to maintain a set angularrotation in a set time-period, thereby providing high positioningresolution of the rotor 20 in this self-sensing (closed-loop) motorcontrol system.

In one implementation of the invention, each gate actuator 42 is aretriggerable-resettable monostable multivibrator (e.g., HEF 4528B,etc.), wherein the “on” duration of the drive signal from themultivibrator is determined by the pulse width control signal from thepulse width control logic 36. In other embodiments, the gate actuators42 may be logic of a microprocessor or microcontroller and/or an ASIC orUECM IC, so long as the pulse width of the drive signal can be madevariable based on the integrated voltage of the back EMF from motor 12.

A power supply 48 provides sufficient DC power (e.g., 5 volts) to motorcontroller 14 and power switches 46, via lines 50 a and 50 b,respectively. Power switches 46 power motor 12 via rails 52 a and 52 b,which represent the electrical connection between power switches 46 andthe motor winding of stator 16. In response to system control signals(e.g., clockwise, counterclockwise, hi/low speed, etc.) from the userinterface 30, each gate actuator 42 selectively activates power switches46, and thus rails 52 a and 52 b, providing rotation speed and directionto motor 12.

As an example, an H-bridge circuit embodies power switches 46 fordriving motor 12. The H-bridge circuit may include a number of powertransistors run by TTL or CMOS logic to selectively connect the windingof motor 12 to power circuit 48, either positive or negative, in orderto effect clockwise or counterclockwise rotation. The H-bridge circuithas an upper, or positive, rail (i.e., rail 52 a), and a negative, orlower, rail (i.e., rail 52 b), supplied by power circuit 48, viarespective power lines 50 a and 50 b for such rotational directioncontrol. To prevent current surges in the reverse direction across theinductive load, fly-back diodes may be used to create a return path forthe current.

In one embodiment, the rails 52 a and 52 b are used in combination withlink capacitors and fly-back diodes, constituting a power supply link,also referred to as a DC link, for providing DC power and current-surgeprotection to the motor winding of stator 16. Such H-bridge circuit maybe provided in IC chips (e.g., L293, L6202, etc.), via a powered MOSFET(e.g., Si9928DY, etc.), or as logic of a microprocessor ormicrocontroller (e.g., MSP430F1491, etc.). Additionally, the H-bridgecircuit may be provided by an ASIC or UECM IC, so long as the poweringand rotational direction of the motor 12 is controllable.

Feedback sampling logic 38 generates output signals received by pulsewidth control logic 36, via line 54. These output signals arerepresentative of the necessary pulse width adjustment or “on” timeadjustment per motor drive pulse. In general, each output signal of thefeedback sampling logic 38 is a voltage related to the back EMF of motor12 generated over a fix sampling window.

With reference to FIG. 4, each motor drive pulse (Mpulse) from the gateactuator 42 switches on the appropriate power switches 46 to power motor12. With motor 12 powered, the motor voltage (Vm) increases until itreaches a steady state during the “on” period for each drive pulse. Whenthe motor drive pulse (Mpulse) transitions to the “off” period, themotor voltage (Vm) decreases. As part of this “off” period, there is aninductive stored energy decay before the generation of the back EMF. Toprevent this inductive stored energy decay from being sampled, a sampledelay (Tdelay) is required. Further to ensure that only the back EMF issampled during this “off” period of the drive pulse (Mpulse), the widthof this sampling window (Swindow) needs also to be defined.

FIG. 2 is a circuit diagram that illustrates one implementation of thefeedback sampling logic 38 according to the present invention whichprovides a sampling delay (Tdelay) and sampling window (S window). Thefeedback sampling logic 38 comprises a switch timing control 56, avoltage amplifier 58, a switch 60, and a sample buffer 62. The voltageamplifier 58 is conventional op-amp (e.g., LT1013 op-amp, LMV922M, etc.)configured to provide as its voltage output the generated back EMF ofthe motor 12. During the “on” time of the motor drive pulse the input tothe voltage amplifier 58, via coupling 53, is either the clockwise orcounterclockwise drive voltages provided to motor 12, via rails 52 a and52 b, respectively. However, during the “off” time of the motor drivepulse, the input to the voltage amplifier 58 will be the inductive loadand the back EMF.

To sample only the back EMF, the output connect of the voltage amplifier58 to the sample buffer 62 is toggled via switch 60 (e.g., 4066bilateral switch) to provide the sampling delay (Tdelay) and samplingwindow (S window). This toggling is control by the switch timing control56, which in the illustrated embodiment comprises two monostablemultivibrators (e.g., 4528). The switch timing control 56 is configuredsuch that the switching of the output state of the first multivibrator57 a triggers the switching of the output state of the secondmultivibrator 57 b. In this arrangement, the ending of the motor drivepulse (Mpulse) triggers the output state of the first multivibrator 57a, which introduces the sampling delay (Tdelay). At the beginning of thenext motor drive pulse (Mpulse), the presence of the motor drive pulse(Mpulse) resets the triggering of the second multivibrator 57 b outputstate.

After expiration of the sampling delay period (Tdelay), which is set bya first timing capacitor 59 a, switching of the output state of thefirst multivibrator 57 a triggers the output state of the secondmultivibrator 57 b. Triggering the output state of the secondmultivibrator 57 b closes switch 60 such that feedback sampling logic 38samples, via sample buffer 62, the back EMF detected by voltageamplifier 58. Switch 60 remains closed until the output state of thesecond multivibrator 57 b is switched by the expiration of the samplingwindow period (Swindow), which is set-also by a second timing capacitor59 b. In one embodiment, the sample delay is set to 0.5 millisecond, andthe sampling window is set to 6.5 milliseconds.

In one embodiment, sample buffer 62 comprises resistors 61, and holdcapacitor 63 in series with a voltage follower 65 a (e.g., LMC6484op-amp, etc.). The voltage follower 65 a (buffer) produces the sampledback EMF voltage stored in the hold capacitor 63 as it outputs andprovides effective isolation of the signal source to avoid loadingeffects. Since the motor 12 may be driven in either a clockwise (CW) ora counterclockwise (CCW) direction, the output of the voltage follower65 a is inverted (e.g., via another LMC6484 op-amp, etc.), to provideboth positive (clockwise) and negative (counterclockwise) back EMFvoltage outputs, via line 54, for use by the pulse width control logic36.

In the exemplary pulse width control logic 36 illustrated by FIG. 3, theintegrated back EMF voltage output of the feedback sampling logic 38 isselectively switched by a related direction control signal (i.e., CW,CCW) of the user interface 30. In this manner, the sampled back EMFvoltage output for the user-selected direction is processed by the pulsewidth control circuit 36. Direction and speed control is provided for bydirection and speed control logic 70 (FIG. 1).

In the embodiment shown by FIG. 3, the direction and speed control logic70 comprises a pair of transistors 67 a and 67 b, one for eachdirection. Each transistor is active when the opposite direction controlsignal (CW or CCW) is selected which permits an associated referencevoltage (Vref) to be added to the sampled back EMF voltage output(Sbemf) for the selected direction and to pull down the opposite sampleback EMF voltage output and associated reference voltage (Vref). Forexample, when a clockwise command direction control signal (CW) isselected at the user interface 30, both the counterclockwise referencevoltage and its associated back EMF voltage output from the feedbacksampling logic 38 are pulled to ground by the related transistor 67 b,driving the input to an associated pulse width control circuit 36 tozero, and vice verse.

An associated potentiometer 71 may be used to manually adjust/controlthe amount of reference voltage (Vref) that is summed with the sampledback EMF voltage output (Sbemf) for variable speed control.Additionally, the motor 12 may be driven at a faster rotational speed byproviding a voltage boosting circuit 73. In the illustrated embodiment,the voltage boosting circuit 73 comprises a pair of transistors 69 a and69 b, one for each direction, which are enabled by a FAST signalselected at the user interface 30. When enabled, the voltage output fromthe feedback sampling logic 38 is boosted by a boost voltage (Vb) to themaximum input voltage for the pulse width control circuit 36. As will beexplained hereafter, boosting the input voltage to the pulse widthcontrol circuit 36 to maximum, drives the motor 12 at a fasterrotational speed under control of the motor controller 14.

For continuous rotation at speeds above about 50 RPMs, additionalcontrol inputs provide power switches 46 with signs to drive the motorat higher RPMs. In one embodiment, lines 44 to the power switches 46 arethe same lines used for both low RPM's and high RPM's, and the change inthe control method is performed internally to the microprocessor in thesoftware or firmware. Such a control method may be either a conventionalpulse width modulated (PWM) speed control method, or PWM combined withEMF, or PWM combined with a sampled back EMF method, wherein the PWMdrive periodically is turned off for short a period of time to obtain aback EMF sample. These control inputs can be from a microprocessor, anASIC or UECM IC.

In the illustrated embodiment of FIG. 3, the pulse width control circuit36 in each rotational direction comprises an integrator 64, and a pulsewidth modulation (PWM) driver 76. Each integrator 64 is an operationalamplifier (i.e., LM6484) which receives the associated sampled back EMFvoltage (Sbemf) output of the feedback sampling logic 38 as one of itsinputs. Each integrator 64 is active when its associate voltage outputfrom the feedback sampling logic 38 is not pulled to ground by directionand speed control logic 70. The other input to each integrator 64 is areference speed control voltage (Vsp), which is derived from a desiredspeed input set by the user interface 30.

As mentioned above, since the motor 12 remains in motion during the“off” periods of the drive pulse due to inertia, the voltage sampledfrom line 53 during this “off” period is the generated rotor back EMF.Accordingly, during this “off” period the input to voltage amplifier 58is the rotor generated back EMF and a control voltage (Vcc) to providethe sampled back EMF (Sbemf) output. Integrating the sampled back EMFvoltage (Sbemf) with integrator 64 provides an indication of rotorspeed, which is then used to adjusting the pulse width of the “on” timeof the next drive current pulse.

The response of the “active” integrator 64 to the input voltage (Sbemf)is a pulse width control signal which drives the pulse width modulationdriving logic 76. In particular, the pulse width control signal sets thepulse width of the “on” time of the motor drive pulse of the gateactuators 42. Accordingly, through the feedback sampling of the back EMFgenerated by the motor 12, the controller 14 increases or decreasesrotor speed by varying the pulse width “on” time of the motor drivepulse such that substantially the same angular distance is covered witheach motor drive pulse. For example, in one embodiment, the “on” timepulse width of the motor drive pulse varies linearly from about 55microseconds at a resulting minimum pulse width control voltage of about0.62 volts to about 800 microseconds at a resulting maximum pulse widthcontrol voltage of about 4.3 volts.

In the illustrated embodiment, current mirror 74 is used such that theactive load from the integrator 64 is capable of driving the PWM driver76 with only the low integrated voltage of the back EMF. The currentmirror 74 comprises a matched pair of transistors having their basecoupled to an associated integrator 64, their emitters coupled to areference voltage, and one of the collectors coupled to its base, andthe other coupled to the PWM driver 76. When the emitters of thetransistors of the current mirror 74 associated with one of theintegrators 64 are pulled low, the current mirror 74 basicallyturns-off, and therefore does not interfere with the PWM driver varyingthe pulse width of the driving signal, driving the motor in the opposeddirection. As such, the current mirror 74 acts as the collector load andprovides a high effective collector load resistance, increasing thegain.

In this arrangement, the voltage output from the feedback sampling logic38 is converted to a current via the current mirror 74. This current isthen provided to the PWM driver 76, which charges a timing capacitor 77to set the width of the drive pulse of the gate actuator 42, wherein thedischarge time is fixed. A frequency generator 78 (e.g., CMOS timer,monostable multivibrator, microprocessor, etc.) is used to provide anaccurate duty cycle frequency. In one embodiment, the frequencygenerator produces a frequency of about 100 hertz.

To enable the motor speed to go to zero, the minimum charge time of thetiming capacitor 77 produces a pulse shorter than the minimum widthneeded to move the motor. At low speeds, the power needed is lower andso the on time is smaller implying that the frequency of pulses could bemade higher. This means that the system could be sampled more often,which gives better control at low speeds. At high speeds, the samplingis less often, producing a linear function in the loop.

In another implementation of the present invention, using amicroprocessor to perform motor controller 14 functions, with theexception of the power switches 46 and amplifier 58, the frequency withwhich the motor is pulsed and the back EMF sampled, represented by thefrequency generator 78, which is now internal to the microprocessor, ismade adjustable. One result is that the motor can be moved even moreslowly while maintaining usable control over its speed by reducing thefrequency with which the motor is pulsed and the back EMF sampled, whileat the same time maintaining a low reference speed control voltage(Vsp). Conversely, a higher frequency can be used, up to the limitationsset by the “on” time of the motor drive pulses, the inductive energydecay time, and the sampling or integration time, for faster controlledmovement, but it becomes more practical to simply increase the referencespeed control voltage (Vsp). Where continuous rotation at high RPM's isrequired, a different control method can be used by the microprocessor.Also, the microprocessor can determine, based on the user inputcommands, which method to use.

Accordingly, with the circuit logic of the present invention, while thetime between pulsing the motor on and sampling the generated voltage isfixed, the motor “on” time is variable to provide a more accurate motorresolution at slow speeds below about 1 RPM. This more accurate motorresolution results from the present invention adjusting the “on” time ofthe next motor drive pulse in order to increase or decrease rotor speedsuch that approximately the same angular distance is covered with eachmotor pulse. In this manner, the present invention accounts fortemperature changes and the variations at different angular positionsdue to the manufacturing process, as well as the uneven wear of motorbearings. Furthermore, the circuit logic of the present inventionenables in one embodiment, such as in a laser level, the positioning ofa beam of electromagnetic energy rotated by the motor rotor within a fewarc-minutes of a desired angular position. The adjusting of the “on”time on the next motor drive pulse according to the sampled andintegrated back EMF according to the circuit logic of the presentinvention, provides accurate, repeatable, and reliable fine positioningoperation to a laser level having a sensorless PM DC motor.

To those skilled in the art, many changes and modifications will bereadily apparent from consideration of the foregoing description of apreferred embodiment without departure from the spirit of the presentinvention. For example, in still another embodiment, the motor drivelogic 34, the pulse width control logic 36, and the feedback samplinglogic 38 may also be implemented, in part or whole, using logic of amicroprocessor or microcontroller (e.g., MSP430F149I) and/or an ASIC orUECM IC, to provide a sampling delay which prevents the sampling of theinductive stored energy of the motor, a sampling window for sampling theback EMF, and to change the pulse width of the “on” time of the motordrive pulse based on the sampled back EMF in order to adjust rotor speedfor high resolution at low speeds. Additionally, it is to be appreciatedthat the descriptions herein and the disclosures hereof are by way ofillustration only and should not be construed as limiting the scope ofthe present invention which is more particularly pointed out by thefollowing claims.

1. A method for providing improved angular resolution to a DC motor, themethod comprising: providing a motor drive pulse having a pulse width tothe motor; providing a sampling window for sampling back EMF of themotor; sampling and integrating said back EMF of the motor in saidsampling window; and changing said pulse width of said motor drive pulsebased on the sampled and integrated back EMF in order to adjust rotorspeed of the motor to maintain a set angular distance in a settime-period.
 2. The method as recited in claim 1 wherein said DC motoris a sensorless permanent magnet DC motor.
 3. The method as recited inclaim 1 wherein said motor drive pulse has an on-time which energizesthe motor, and an off-time diring which the motor is deenergized.
 4. Themethod as recited in claim 3 further comprising providing a samplingdelay following said on-time of said motor drive pulse which preventssampling of inductive stored energy of the motor.
 5. The method asrecited in claim 1 further comprising providing before said samplingwindow a sampling delay, wherein said sampling delay begins at anoff-time of said motor drive pulse and ends about 0.5 millisecondthereafter.
 6. The method as recited in claim 1 wherein said samplingwindow is set to about 6.5 milliseconds.
 7. The method as recited inclaim 1 wherein said sampled and integrated back EMF is compared to areference voltage to determine an amount of said change to an on-time ofsaid pulse width.
 8. The method as recited in claim 1 wherein theangular resolution is in the range of about 2 to about 5 arc minutes. 9.A method of providing improved angular resolution at rotational speedsbelow about 1 rpm to a rotor of a sensorless permanent magnet DC motorused to move and position a beam of electromagnetic energy, said methodcomprising: providing a sampling delay which prevents sampling ofinductive stored energy of the motor; providing after said samplingdelay, a sampling window for sampling back EMF of the motor; integratingsaid sampled back EMF; and changing a pulse width of a motor drive pulsecontrolling rotation of the motor based on said sampled back EMF inorder to adjust rotor speed to maintain a set angular distance in a settime-period.
 10. The method as recited in claim 9 further comprisingsupplying the motor said motor drive pulse, wherein said pulse width hasan “on” time which imparts rotor motion.
 11. The method as recited inclaim 9 further comprising supplying the motor said motor drive pulse,wherein said pulse width has an “on” time which imparts rotor motion,and wherein said sampling delay prevents sampling of inductive storedenergy of the motor after expiration of said “on” time.
 12. The methodas recited in claim 9 wherein said sampling delay lasts about 0.5millisecond.
 13. The method as recited in claim 9 wherein said samplingwindow is about 6.5 milliseconds.
 14. The method as recited in claim 9wherein said integrated back EMF is compared to a reference voltage todetermine amount of said change to said pulse width.
 15. The method asrecited in claim 9 wherein said electromagnetic energy is laser light.16. The method as recited in claim 6, wherein the angular resolution isin the range of about 2 to about 5 arc minutes.
 17. A motor controllerfor driving a sensorless permanent magnet DC motor with a motor drivepulse and providing an improved angular resolution for rotor pointing,the motor controller comprising: feedback sample logic adapted tomeasure back EMF generated by the motor, said feedback sample logicproviding a sampling delay which prevents sampling of inductive storedenergy of the motor, and a sampling window for sampling said back EMF ofthe motor after said sampling delay, and pulse width control logicadapted to integrate and compare sampled back EMF to a reference voltageto determine an amount of change to a pulse width of the motor drivepulse in order to adjust rotor speed to maintain a set angular distancein a set time-period.
 18. The motor controller as recited in claim 17wherein said sampling delay lasts about 0.5 millisecond.
 19. The motorcontroller as recited in claim 17 wherein said sampling window is about6.5 milliseconds.
 20. The motor controller as recited in claim 17wherein the angular resolution is in the range of about 2 to about 5 arcminutes.
 21. The motor controller as recited in claims 17 wherein saidmotor controller is implemented with elements selected from the groupcomprising circuitry components, transistor circuits, programmable logiccircuits, a microprocessor, a microcontroller, an application specificintegrated circuit an universal electronically commutated motorintegrated circuit, operational amplifier circuits, analog switches, andcombinations thereof.
 22. The motor controller as recited in claims 17wherein said pulse width has an on-time which imparts rotor motion. 23.The motor controller as recited in claims 17 further comprising a motordrive logic having power switches adapted to energize at least onewinding of the motor and being controlled by said motor drive pulse. 24.The motor controller as recited in claim 17 wherein said sampling delayis trigger by said motor drive pulse.
 25. The motor controller asrecited in claim 17 further comprising circuit logic for signalamplification of the generated back EMF.
 26. The motor controller asrecited in claim 17 wherein said pulse width control logic furtherincludes a current mirror coupled between an integrator receiving saidsample back EMF and a timing capacitor used to set an “on” time of saidmotor drive pulse based on said reference voltage.
 27. The motorcontroller as recited in claim 17 further comprising a direction andspeed control logic adapted to control rotation direction and rotationspeed of the motor.
 28. The motor controller as recited in claim 27wherein said direction and speed control logic further include speedboost logic adapted to boost said reference voltage to a maximum valuein order to rotate the motor at a faster rotational speed.
 29. The motorcontroller as recited in claim 27 wherein said direction and speedcontrol logic including a potentiometer to manually adjust/control anamount of said reference voltage for variable speed control.
 30. Themotor controller as recited in claims 17 provided in a laser levelhaving the sensorless permanent magnet DC motor.